1. Field of the Invention
The present invention is directed to a new class of rigid rod and ladder benzobisazole polymers. The present invention is further directed to a process for making said polymers. The subject invention is still further directed to a process for producing light utilizing a rigid rod and ladder polymer. Finally, the subject invention encompasses light emitting devices utilizing rigid rod and ladder polymers of the class useful in the process of producing light.
2. Background of the Prior Art
Optoelectronics has rapidly grown in importance in recent years because of its impact on modern information processing systems and optical communication. The kinds of devices in which optoelectronics plays a major role includes light emitting diodes (LEDs) for displays, lasers, photo detectors, electrophotographic imaging, solar cells, waveguide switches, modulators and the like. Obviously, these types of devices are at the cutting edge of technology. The focus of the present invention lies on that area of optoelectronic technology which involves LEDs and other light emitting devices.
Light emitting diodes of the prior art were fabricated of inorganic semiconductors. Unfortunately, inorganic semiconductor optoelectronic materials, although useful in producing LEDs, have major shortcomings. These inorganic semiconductors, which usually are made of alloys of elements of Groups III and V of the Periodic Table of the Elements, are very high cost materials which are not easily processed into optoelectronic materials. Indeed, these inorganic semiconductor materials are incompatible with each other. Another shortcoming of an LED fabricated of an inorganic material is its failure to provide a wide spectral range. Thus, multicolor emitters for full color flat-paneled displays, essential for applications such as computer and television screens, is precluded. Even a known exception to this rule, the inorganic semiconductor material LiNbO.sub.3 which is not subject to this limitation, cannot be used in an LED. LiNbO.sub.3, as well as other related materials which do not suffer from the problem associated with narrow spectral range, does not emit light.
Because of the failings of inorganic semiconductor materials in optoelectronic devices, focus has shifted to organic materials. Thus, significant research has been invested in the development of electrically conducting polymers. Such activity has been accelerated with the development of polymer doping. However, up to the present time, the only polymer which has found commercial acceptance as a conducting polymer is polyaniline. Polyaniline is utilized as an electrode material in batteries.
The development of conducting polymers has led to the investigation of polymers for utilization in this application. Of particular interest are the so-called conjugated rigid-rod and ladder polymers. These polymers are mentioned in the prior art as electronic, photoelectronic and non-linear optical materials. A plurality of applications and patents have issued disclosing such polymers. None of these references, however, have identified such polymers as light emitting devices or their suitability as a light emitting source in an LED.
A plurality of U.S. patents have issued to Wolfe and others which describe the synthesis, characterization and fabrication of extended chain or rod-like polymers. Among these disclosures is U.S. Pat. No. 4,225,700 which is directed to the specific polymer, poly((benzo(1,2-d:4,5-d')-bisthiazole-2,6-diyl)1,4-phenylene) (PBT). The '700 patent, moreover, sets forth another extended chain polymer, poly((benzo(1,2-d:5,4-d')bisoxazole-2,4-diyl)-1,4-phenylene) (PBO). The polybenzobisthiazoles claimed in the '700 patent are employed in the formation of liquid crystalline solution compositions in which the solute polymer is present in a concentratrion of no more than 10% by weight. Other disclosures by Wolfe et al. of related polymers in this class include U.S. Pat. Nos. 4,533,692; 4,533,724; 4,703,103; 4,772,678.
The article, Wolfe et al., Macromolecules, 14, 915-920 (1981) sets forth the synthesis of PBO, disclosed as prior art in the aforementioned '700 patent. The synthesis of the compound claimed in the '700 patent, PBT, is set forth in the article, Wolfe et al., Macromolecules, 14, 915-920 (1981). The aforementioned PBT polymer is investigated in terms of its third-order non-linear optical properties in Vanherzeele et al., Appl. Phys. Lett., 58, 663-665. It is noted that in this paper PBT is denoted as PBZT. Rao et al., Appl. Phys. Lett., 48, 1187-1189 (1986) discusses the third-order non-linear optical susceptibility of PBT.
Additional papers which employ PBT, including solubilization, complexation and vibrational spectroscopic characterization of PBT and related polymers, are presented in Jenekhe et al., Macromolecules, 22, 3216-3222 (1989); Jenekhe et al., Macromolecules, 23, 4419-4429 (1990); and Shen et al., Polymer, 23, 969-973 (1982).
Yokoyama et al., Chem. Lett. Chem. Soc. Jap. 779-782 (1990) describes the synthesis of ultrathin films of poly(vinylene benzothiazole) (PVBT) which are recited to have potential as electronic andoptical devices. A later disclosure, Oshaheni et al., Proc. ACS. Div. Pol. Mat.: Sci. and Eng., 67, 474-475 (1992) (Query: When was this paper published?) indicates that the polymer synthesized by Yokoyama et al. was not PVBT. The Osaheni et al. paper describes the preparation of four conjugated polybenzobisthiazoles: polybenzobisthiazole (PBBT), poly(benzobisthiazole vinylene) (PBVT), poly(phenylene benzobisthiazole) (PBZT) and poly(benzobisthiazole divinylene) (PBTDV). It is appreciated that the synthesis of PBZT was well known in the art at the time of its synthesis by Osaheni et al.
The above extensive discussion emphasizes the developing interest in conjugated rigid-rod polymers, especially benzobisthiazole polymers. However, nothing in the art directed to rigid-rod polymers suggests their employment in light emitting diodes and other light emitting devices. This is not to say that such devices employing organic electroluminescence agents were not known in the art. U.S. Pat. No. 5,142,343 to Hosokawa et al. is directed to organic electroluminescence devices which comprise an organic semiconductor zone and an organic insulator zone sandwiched between electrodes. The organic insulator zone comprises an organic light emitting area and the organic semiconductor zone includes an electroconducting oligomer. The electroconducting material is preferably a thiothene-containing oligomer. The critical light emitting area, the organic insulator zone, is a polycyclic condensation aromatic compound. These compounds, structurally distinguished from the rigid-rod polymers discussed above, are further differentiated in that they are compounds rather than polymers.